Deflector devices

ABSTRACT

A deflector device for use with a tow line between a seismic survey vessel and a tow, in particular a streamer or streamer array, in the water behind the vessel comprises a vertically oriented wing-shaped body for producing a sideways force as it is towed through the water, and a towing bridle adapted to connect the wing-shaped body to the tow line. The bridle comprises first and second connecting elements connected between the tow line and respective longitudinally-spaced points along the high pressure side of the wing-shaped body. The wing-shaped body includes one or more buoyancy elements to render it slightly positively buoyant, and the length of at least one of the connecting elements is adjustable by remote contol in order to tilt the wing-shaped body. This gives the sideways force a vertical component, and so allows remote control of the depth of the deflector device, as well as its lateral offset from the vessel.

This invention relates to deflector devices of the kind used between atowing vessel and a tow located in water, for example a seismic streameror streamer array, or a seismic source array, in order to pull the towout to one side of the vessel, so as to position it at a desired lateraloffset from the course followed by the vessel.

DESCRIPTION OF THE RELATED ART

A deflector device of the kind used between a towing vessel and a towlocated in water is described in detail in our U.S. Pat. No. 5,357,892,and comprises a wing-shaped deflector body having a remotely-operablepivotal lever or “boom” which extends rearwardly from a point near themiddle of the trailing edge of the wing-shaped body. In use, thewing-shaped body is suspended beneath a float so as to be completelysubmerged and positioned generally vertically in the water, and isconnected to the towing vessel by means of a tow line, while the tow isconnected to the end of the boom remote from the wing-shaped body. Asthe device is pulled through the water, the wing-shaped body produces asideways force, or “lift”, which moves the tow laterally. This lift canbe varied by adjusting the angle of the boom from the vessel, thuspermitting the lateral offset of the tow from the course of the vesselto be varied in use.

The deflector device of U.S. Pat. No. 5,357,892 has been successfullycommercialised by the Applicant as its MONOWING deflector device. Inuse, rolling stability of the device is provided by the connection tothe float, while stability of the device about a vertical axis isprovided by the drag produced by the tow.

The MONOWING deflector devices in current use are very large, typically7.5 m high by 2.5 m wide, and weigh several tonnes. They are usuallysuspended around 2 m to 8 m below the float by means such as a fibrerope, and are also provided with a safety chain intended to preventseparation of the float and wing-shaped body in the event that the ropebreaks. In rough weather, the upper part of the wing-shaped body mayrise up out of the water, allowing the rope connecting the wing-shapedbody and the float to go slack. If the wing-shaped body then dropsabruptly, the rope, and possibly even the safety chain, may break,and/or their attachment points on the wing-shaped body may be badlydamaged.

Additionally, the depth at which the current deflector device operatesis effectively determined by the length of the rope connecting it to thefloat. As a result of this, the operating depth of the deflector devicecannot readily be varied while the device is deployed in the water. Andsince the normal operating depth of the current deflector device istypically a few meters, in the event of the onset of bad weather, thedevice and all the streamers and other equipment directly or indirectlyattached to it have to be recovered onto the towing vessel.

It is an object of the present invention to alleviate the drawbacksarising from the connection of the deflector device to the float.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a deflector devicefor use with a tow line between a towing vessel and a tow in waterbehind the vessel, the device comprising a wing-shaped body, and atowing bridle adapted to connect the wing-shaped body to the tow line,the bridle comprising first and second connecting elements havingrespective first ends connected to respective longitudinally-spacedpoints along the high pressure side of the wing-shaped body andrespective second ends adapted to be coupled to the tow line, and thewing-shaped body being shaped to produce in use a sideways force whichurges the tow line laterally with respect to the direction of movementof the towing vessel, further comprising one or more buoyancy elementsdisposed within and/or secured to the upper end of the wing-shaped body,and remotely-operable means for adjusting the length of at least one ofthe connecting elements in order to tilt the wing-shaped body so as togive said sideways force a vertical component, whereby to control thedepth of the deflector device as well as its lateral offset from thevessel.

It will be appreciated that since the deflector device of the inventioncan generate a controllable vertical force, this force, together withthe buoyancy of the one or more buoyancy elements, can be selected andadjusted so that the separate surface float is no longer required, andthe operating depth of the device can be remotely controlled while thedevice is deployed in the water. In particular, at the onset of badweather, the deflector device and its tow can be caused to dive to agreater depth, where the effects of the bad weather are much reduced,until the weather improves.

Advantageously, the one or more buoyancy elements has or have a buoyancyselected to give the complete device a small positive buoyancy.

The remotely-operable adjusting means preferably comprises a telescopicmember, which may be hydraulically actuated, connected in series in oneof the connecting elements, which are advantageously titanium chains.

In a first implementation of the invention, the deflector device furthercomprises a boom extending rearwardly from the wing-shaped body, the endof the boom remote from the wing-shaped body being connected, in use, tothe tow, and remotely-operable means for adjusting the angle between theboom and the wing-shaped body to vary the sideways force produced by thewing-shaped body.

In a second implementation of the invention, the deflector devicefurther comprises a boom extending rearwardly from the wing-shaped body,an auxiliary wing-shaped body, smaller than the firstmentioned (orprincipal) wing-shaped body, secured to the end of the boom remote fromthe principal wing-shaped body and shaped so as to produce in use asideways force in generally the opposite direction to that produced bythe principal wing-shaped body, and remotely-operable means foradjusting the angle between the boom and the principal wing-shaped bodyto vary the sideways force produced by the principal wing-shaped body.

In a third and preferred implementation of the invention, the deflectordevice further comprises a boom extending rearwardly from thewing-shaped body, an auxiliary wing-shaped body, smaller than thefirstmentioned (or principal) wing-shaped body, secured to the end ofthe boom remote from the principal wing-shaped body and shaped so as toproduce in use a sideways force in generally the opposite direction tothat produced by the principal wing-shaped body, and remotely-operablemeans for varying the angle of the auxiliary wing-shaped body to varythe sideways force produced by the auxiliary wing-shaped body, andthereby vary the sideways force produced by the principal wing-shapedbody.

Advantageously, the auxiliary wing-shaped body is provided with atrailing edge flap angled away from the boom, typically at about 35°.

The invention also includes a method of performing a marine seismicsurvey, the method including towing a plurality of laterally spacedseismic steamers over an area to be surveyed, wherein the lateralposition and the depth of at least one of the streamers are controlledby a deflector device in accordance with any one of the precedingstatements of invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only, withreference to the accompanying drawings, of which:

FIG. 1 is a somewhat schematic view of a seismic survey vessel carryingout a marine seismic survey;

FIG. 2 is a somewhat schematic part-sectional view of a first embodimentof a deflector device in accordance with the present invention, for usein carrying out the survey of FIG. 1;

FIGS. 3A and 3B are respective perspective views of the deflector deviceof FIG. 2;

FIG. 3C is a more detailed view of part of the deflector device of FIG.2:

FIG. 4A is a somewhat schematic part-sectional view of a secondembodiment of a deflector device in accordance with the presentinvention, for use in carrying out the survey of FIG. 1; and

FIGS. 4B and 4C show different operating positions of part of thedeflector device of FIG. 4A.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The seismic survey vessel shown in FIG. 1 is indicated generally at 10,and is preferably as described in our PCT Patent Application No.PCT/GB98/01832 (WO 99/00295). The vessel 10 is shown towing a seismicsource 15, typically a TRISOR multiple air gun source of the kinddescribed in our U.S. Pat. No. 4,757,482, and an array 16 of foursubstantially identical streamers 18. However, it will be appreciatedthat, in practice, many more than four streamers can be towed, forexample by using the techniques described in our PCT Patent ApplicationNo. PCT/IB98/01435 (WO 99/15913). The streamers 18 are towed by means oftheir respective lead-ins 20 (ie the high strength steel- orfibre-reinforced electrical or electro-optical cables which conveyelectrical power, control and data signals between the vessel 10 and thestreamers), and their spread is controlled by two deflector devices,indicated at 22, connected to the respective forward ends 24 of the twooutermost streamers. The deflector devices 22 act in co-operation withrespective spreader lines 26 connected between the forward end 24 ofeach outermost streamer 18 and the forward end 24 of its adjacentstreamer to maintain a substantially uniform spacing between thestreamers.

One of the deflector devices 22 is shown in section in FIG. 2. Thedeflector device 22 is similar in general principle to the deflectordevice of our U.S. Pat. No. 5,357,892, but is a much improved version ofit. In particular, the deflector device 22 has a main wing-shaped body28 which is coupled in use to a respective outer lead-in 20, and whichcorresponds to the deflector body 2 of U.S. Pat. No. 5,357,892. However,the main wing-shaped body 28 is of improved hydrodynamic cross-sectionalshape and includes a fixed-angle trailing edge flap 29, both of whichfeatures enhance lift. Also, the main wing-shaped body 28 is providedwith vortex controlling end plates 30 (see FIGS. 3A and 3B) of the kinddescribed in our PCT Patent Application No. PCT/FR99/02272, to reducedrag and improve stability, and is largely made of titanium to reduceweight.

Additionally, the angle lever 10 of U.S. Pat. No. 5,357,892 is replacedby a rearwardly extending fixed angle boom 32, which is detachablyconnected at one end 34 to the low pressure side 36 of the body 28 nearthe trailing edge flap 29, at a mounting bracket 38. The boom 32 is ofsandwich construction, and is made from two similarly shaped plates 39which are bolted together at intervals along their length and whichsandwich between them the mounting bracket 38. Typically, the boom 32 isdetached whenever the deflector device 22 is on the vessel 10, for easeof stowage. The other end 40 of the boom 32 has a towing eye 42, coupledin use to the forward end 24 of a respective one of the two outermoststreamers 18.

An auxiliary wing-shaped body 44, which is much smaller than the body 28in length, thickness and chord, is pivotally secured as will beexplained hereinafter to the end 40 of the boom 32, with itslongitudinal axis (which lies in a plane perpendicular to the plane ofFIG. 2) extending parallel to the longitudinal axis of the body 28. Theshape of the body 44 is designed to produce, in use, a sideways force ina direction approximately opposite to that produced by the body 28(approximately opposite, because as will become apparent, the directionof the force varies in use). This sideways force is increased byproviding the body 44 with a fixed trailing edge flap 46, angled awayfrom the boom 32 at an angle of about 35°.

As best seen in FIGS. 3A and 3B, the auxiliary wing-shaped body 44 isimplemented in two symmetrical halves 44 a and 44 b, which each havevortex-reducing end plates 45 and which are disposed on opposite sidesof the boom 32. The two halves 44 a, 44 b of the auxiliary wing-shapedbody 44 are rotatable in unison about a common axis perpendicular to theplane of the boom 32, so as to vary the angle of the chord of theauxiliary wing-shaped body 44 with respect to the boom. Rotation of theauxiliary wing-shaped body 44 is effected by a telescopic actuator 48pivotally mounted between the plates 39 of the boom 32, the actuatorbeing pivotally connected to a lever arm or eccentric 47 attached to thetwo halves 44 a and 44 b of the auxiliary wing-shaped body 44 (see FIG.3C). The telescopic actuator 48 is operated from a remotely-controllableelectro-hydraulic control pack 49, which is also mounted between theplates 39 of the boom 32.

It will be appreciated that varying the angle of the auxiliarywing-shaped body 44 of the deflector device 22 changes the angle of themain wing-shaped body 28 with respect to the direction of tow, and sochanges the lift produced by the main wing-shaped body. This in turnchanges the lateral offset produced by the deflector device 22.

In accordance with the present invention, the deflector device 22 ismade approximately neutrally buoyant, by including gas-filled pipe-likebuoyancy elements 58 extending longitudinally within it from top tobottom, and/or by providing an integral buoyancy element at its upperend similar to but smaller than that described in our co-pending UnitedKingdom Patent Application Nos. 0023775.0, 0025719.6 & 0029451.2. Inpractice, the deflector device 22 is preferably designed to be slightlypositively buoyant, so that in the event of a malfunction, ittends-to-float-rather than sink. Additionally, the main wing-shaped body28 of the deflector device 22 is coupled to the respective lead-in 20 bya towing bridle 50 comprising two titanium chains 52 and 54, the chain54 having a remotely operable, hydraulically actuated, telescopic strut56 connected in series in it.

With the telescopic strut 56 in its mid-length position, the combinedlength of the chain 54 and the strut 56 is substantially equal to thelength of the chain 52, which tends to hold the main wing-shaped body 28in a substantially vertical attitude in the water, so that substantiallyall the force or “lift” generated by it is directed sideways, as in theprior art MONOWING deflector device, but with just enough of a downwardcomponent to counteract the slightly positive buoyancy mentioned above.However, changing the length of the strut 56 tends to tilt the mainwing-shaped body 28 away from the vertical, so giving the sideways forcegenerated by it a more significant vertical component in the upward ordownward direction, and thus permitting the depth of the device to bevaried.

It will be appreciated that as a result of making the deflector device22 approximately neutrally buoyant and capable of generating aremotely-controllable vertical force, a separate surface float is nolonger required, and the operating depth of the device can be remotelycontrolled while the device is deployed in the water. In particular, inthe event of the onset of bad weather, the deflector device 22 and thestreamers 18 attached to it can be caused to dive to a greater depth,where the effects of the bad weather are much reduced, until the badweather passes.

FIGS. 4A to 4C show at 60 an alternative embodiment of the deflectordevice 22 of FIGS. 2 and 3A to 3C, with corresponding parts having thesame reference numbers as were used in FIGS. 2 and 3A to 3C. Theprincipal difference between this alternative embodiment and theembodiment of FIGS. 2 and 3A to 3C is that in the deflector device 60,the boom 32 is pivotally connected to the low pressure side 36 of themain wing-shaped body 28 at the mounting bracket 38, while the auxiliarywing-shaped body 44 is fixedly secured at or near the midpoint of itstrailing edge 62 to the end 40 of the boom 32, with its leading edge 64inclined away from the body 28 such that the chord of the body 44 isinclined at an angle of about 10° to the boom.

Pivotal movement of the boom 32 is controlled by a mechanism comprisingfirst and second struts 66, 68, which are pivotally connected to eachother at 70 and to each end of the boom at 71 a and 71 b, forming withthe boom a triangle, and an extending hydraulic actuator strut 72pivotally connected between the apex of the triangle, ie the pivotalconnection point 70 of the struts 66, 68, and a pivotal connection point74 positioned on the low pressure side 36 of the body 28 between itsmidpoint and its trailing edge. The actuator strut 72 is connected to beoperated by a remotely-operable hydraulic control system (not shown)disposed within the body 28.

It will be appreciated that extension of the hydraulic actuator strut72, from its unextended position of FIG. 4A, will move the boom 32outwardly from the low pressure side 36 of the body 28, from its closestposition shown in FIG. 4A. The extent of the outward movement ispreferably about 209, as shown in FIGS. 4B and 4C.

As the boom 32 is pivoted away from the body 28, the sideways forceproduced by the body 44 acts as a restoring force, and thus varies theangle of the body 28 with respect to the direction of tow, so changingthe lift produced by the body 28. This restoring force augments therestoring force produced by the drag of the towed streamer 18 (and inparticular, reduces the effect of any stability-reducing variations orreductions in that drag). Indeed, the deflector device 60 will remainstable with no streamer attached, eg if its streamer 18 breaks or issevered at its forward end 24 (this is also true for the deflectordevice 22 of FIGS. 2 and 3A to 3C).

It will be appreciated that many modifications can be made to thedescribed embodiments of the invention.

In particular, the titanium chains 52, 54 of the towing bridle 50 can bereplaced by cables made from high strength fibres, eg Kevlar fibres,while the telescopic strut 56 can be replaced by any other suitablehydraulic or electric mechanism for changing the relative lengths of thechains or cables, which mechanism can be housed inside the body 28 andarranged to retract or pay out one or both of the chains or cables. Andthe auxiliary wing-shaped body 44 can be made from a plastics materialreinforced with high strength fibres, eg Kevlar fibres, and, in thedeflector device 22, electrically operated rather than operated by thehydraulic actuator 48.

Additionally, the devices 22 and 60 can be used with tows other thanstreamers, for example seismic sources, and the tow need not beconnected to the end 40 of the boom 32 (it could instead be connected tothe lead-in 20, at a point near where the bridle 24 is connected to thelead-in). Also, the invention can if desired be used with a deflectordevice like that described in our U.S. Pat. No. 5,357,892, ie adeflector device without the auxiliary wing-shaped body 44.

Finally, although the invention has been described in relation todeflector devices whose lift can be varied by varying the angle of thedevice with respect to the direction of tow, it is also applicable inits broadest aspect to a fixed angle deflector device, eg of the kindreferred to as a “door”.

1. A deflector device for use with a tow line between a towing vesseland a tow in water behind the vessel, the device comprising awing-shaped body, and a towing bridle adapted to connect the wing-shapedbody to the tow line, the bridle comprising first and second connectingelements having respective first ends connected to respectivelongitudinally-spaced points along the high pressure side of thewing-shaped body and respective second ends adapted to be coupled to thetow line, and the wing-shaped body being shaped to produce in use asideways force which urges the tow line laterally with respect to thedirection of movement of the towing vessel, further comprising at leastone buoyancy element disposed within the upper end of the wing-shapedbody, and remotely-operable means for adjusting the length of at leastone of the connecting elements in order to tilt the wing-shaped body soas to give said sideways force a vertical component, whereby to controlthe depth of the deflector device as well as its lateral offset from thevessel.
 2. A deflector device as claimed in claim 1, wherein said atleast one buoyancy element has a buoyancy selected to give the deflectordevice a small positive buoyancy.
 3. A deflector device as claimed inclaim 1, wherein the remotely-operable adjusting means comprises atelescopic member connected in series in one of the connecting elements.4. A deflector device as claimed in claim 3, wherein the telescopicmember is hydraulically operated.
 5. A deflector device as claimed inclaim 1, wherein the connecting elements are chains.
 6. A deflectordevice as claimed in claim 5, wherein the chains are titanium chains. 7.A deflector device as claimed in claim 1, further comprising a boomextending rearwardly from the wing-shaped body, the end of the boomremote from the wing-shaped body being connected, in use, to the tow,and remotely-operable means for adjusting the angle between the boom andthe wing-shaped body to vary the sideways force produced by thewing-shaped body.
 8. A deflector device as claimed in claim 1, furthercomprising a boom extending rearwardly from the wing-shaped body, anauxiliary wing-shaped body, smaller than the principal wing-shaped body,secured to the end of the boom remote from the principal wing-shapedbody and shaped so as to produce in use a sideways force in generallythe opposite direction to that produced by the principal wing-shapedbody, and remotely-operable means for adjusting the angle between theboom and the principal wing-shaped body to vary the sideways forceproduced by the principal wing-shaped body.
 9. A deflector device asclaimed in claim 1, further comprising a boom extending rearwardly fromthe wing-shaped body, an auxiliary wing-shaped body, smaller than theprincipal wing-shaped body, secured to the end of the boom remote fromthe principal wing-shaped body and shaped so as to produce in use asideways force in generally the opposite direction to that produced bythe principal wing-shaped body, and remotely-operable means for varyingthe angle of the auxiliary wing-shaped body to vary the sideways forceproduced by the auxiliary wing-shaped body, and thereby vary thesideways force produced by the principal wing-shaped body.
 10. Adeflector device as claimed in claim 8, wherein the auxiliarywing-shaped body is provided with a trailing edge flap angled away fromthe boom.
 11. A deflector device as claimed in claim 10, wherein theauxiliary wing-shaped body is provided with a trailing edge flap angledaway from the boom at about 35°.
 12. A method of performing a marineseismic survey, the method comprising: towing a plurality of laterallyspaced seismic steamers over an area to be surveyed; and controlling alateral position and a depth of at least one of the streamers using adeflector device for use with a tow line between a towing vessel and atow in water behind the vessel, the deflector device comprising awing-shaped body, and a towing bridle adapted to connect the wing-shapedbody to the tow line, the bridle comprising first and second connectingelements having respective first ends connected to respectivelongitudinally-spaced points along the high pressure side of thewing-shaped body and respective second ends adapted to be coupled to thetow line, and the wing-shaped body being shaved to produce in use asideways force which urges the tow line laterally with respect to thedirection of movement of the towing vessel, further comprising at leastone buoyancy element disposed within the upper end of the wing-shapedbody, and remotely-operable means for adjusting the length of at leastone of the connecting elements in order to tilt the wing-shaped body soas to give said sideways force a vertical component, whereby to controlthe depth of the deflector device as well as its lateral offset from thevessel.
 13. A method as claimed in claim 12, wherein the deflectordevice comprises at least one buoyancy element, and further comprisingselecting a buoyancy of said at least one buoyancy element to give thedeflector device a small positive buoyancy.
 14. A method as claimed inclaim 12, wherein controlling the lateral position and the depthcomprises adjusting a telescopic member connected in series in one ofthe connecting elements.
 15. A method as claimed in claim 12, furthercomprising a boom extending rearwardly from the wing-shaped body, theend of the boom remote from the wing-shaped body being connected, inuse, to the tow, and wherein controlling the lateral position and thedepth comprises adjusting the angle between the boom and the wing-shapedbody to vary the sideways force produced by the wing-shaped body.
 16. Adeflector device for use with a tow line between a towing vessel and atow in water behind the vessel, the device comprising a wing-shapedbody, and a towing bridle adapted to connect the wing-shaped body to thetow line, the bridle comprising first and second connecting elementshaving respective first ends connected to respectivelongitudinally-spaced points along the high pressure side of thewing-shaped body and respective second ends adapted to be coupled to thetow line, and the wing-shaped body being shaped to produce in use asideways force which urges the tow line laterally with respect to thedirection of movement of the towing vessel, further comprising at leastone buoyancy element secured to the upper end of the wing-shaped body,and remotely-operable means for adjusting the length of at least one ofthe connecting elements in order to tilt the wing-shaped body so as togive said sideways force a vertical component, whereby to control thedepth of the deflector device as well as its lateral offset from thevessel.
 17. A deflector device as claimed in claim 16, wherein said atleast one buoyancy element has a buoyancy selected to give the deflectordevice a small positive buoyancy.
 18. A deflector device as claimed inclaim 16, wherein the remotely-operable adjusting means comprises atelescopic member connected in series in one of the connecting elements.19. A deflector device as claimed in claim 18, wherein the telescopicmember is hydraulically operated.
 20. A deflector device as claimed inclaim 16, wherein the connecting elements are chains.
 21. A deflectordevice as claimed in claim 20, wherein the chains are titanium chains.22. A deflector device as claimed in claim 16, further comprising a boomextending rearwardly from the wing-shaped body, the end of the boomremote from the wing-shaped body being connected, in use, to the tow,and remotely-operable means for adjusting the angle between the boom andthe wing-shaped body to vary the sideways force produced by thewing-shaped body.
 23. A deflector device as claimed in claim 16, furthercomprising a boom extending rearwardly from the wing-shaped body, anauxiliary wing-shaped body, smaller than the principal wing-shaped body,secured to the end of the boom remote from the principal wing-shapedbody and shaped so as to produce in use a sideways force in generallythe opposite direction to that produced by the principal wing-shapedbody, and remotely-operable means for adjusting the angle between theboom and the principal wing-shaped body to vary the sideways forceproduced by the principal wing-shaped body.
 24. A deflector device asclaimed in claim 16, further comprising a boom extending rearwardly fromthe wing-shaped body, an auxiliary wing-shaped body, smaller than theprincipal wing-shaped body, secured to the end of the boom remote fromthe principal wing-shaped body and shaped so as to produce in use asideways force in generally the opposite direction to that produced bythe principal wing-shaped body, and remotely-operable means for varyingthe angle of the auxiliary wing-shaped body to vary the sideways forceproduced by the auxiliary wing-shaped body, and thereby vary thesideways force produced by the principal wing-shaped body.
 25. Adeflector device as claimed in claim 24, wherein the auxiliarywing-shaped body is provided with a trailing edge flap angled away fromthe boom.
 26. A deflector device as claimed in claim 25, wherein theauxiliary wing-shaped body is provided with a trailing edge flap angledaway from the boom at about 35°.
 27. A method of performing a marineseismic survey, the method comprising: towing a plurality of laterallyspaced seismic steamers over an area to be surveyed; and controlling alateral position and a depth of at least one of the streamers using adeflector device for use with a tow line between a towing vessel and atow in water behind the vessel, the device comprising a wing-shapedbody, and a towing bridle adapted to connect the wing-shaped body to thetow line, the bridle comprising first and second connecting elementshaving respective first ends connected to respectivelongitudinally-spaced points along the high pressure side of thewing-shaped body and respective second ends adapted to be coupled to thetow line, and the wing-shaped body being shaped to produce in use asideways force which urges the tow line laterally with respect to thedirection of movement of the towing vessel, further comprising at leastone buoyancy element secured to the upper end of the wing-shaped body,and remotely-operable means for adjusting the length of at least one ofthe connecting elements in order to tilt the wing-shaped body so as togive said sideways force a vertical component, whereby to control thedepth of the deflector device as well as its lateral offset from thevessel.
 28. A method as claimed in claim 27, wherein the deflectordevice comprises at least one buoyancy element, and further comprisingselecting a buoyancy of said at least one buoyancy element to give thedeflector device a small positive buoyancy.
 29. A method as claimed inclaim 27, wherein controlling the lateral position and the depthcomprises adjusting a telescopic member connected in series in one ofthe connecting elements.
 30. A method as claimed in claim 27, furthercomprising a boom extending rearwardly from the wing-shaped body, theend of the boom remote from the wing-shaped body being connected, inuse, to the tow, and wherein controlling the lateral position and thedepth comprises adjusting the angle between the boom and the wing-shapedbody to vary the sideways force produced by the wing-shaped body.